1. Field of the Invention
The present invention relates to a neutral density (ND) filter and a method of producing the same, and more particularly, to a reduction in reflectance of a front surface and a rear surface of an ND filter and a stable production thereof.
2. Description of the Related Art
Up to now, in a photographing optical system, when a luminance of a subject is too high, even when a diaphragm is closed (an aperture diameter is set to be small), a predetermined amount or more of light may enter on a photosensitive plane. Therefore, an ND filter is mounted in a portion of the photographing optical system, thereby regulating the amount of light entering on the photosensitive plane.
Spectral characteristics of the ND filter in this case are required to have a uniform transmittance over an entire visible region for the purpose of merely reducing the amount of incident light. As the ND filter, a glass filter obtained by dissolving an absorbing material in glass (transparent substrate), a sheet filter formed in a film shape containing a pigment, and the like are used in many cases.
However, those ND filters have problems in terms of durability, that is, the spectral characteristics are not uniform (neutrality) over the entire visible region, the spectral characteristics change after a long-term use, and abnormality occurs in an outer appearance.
As a method of solving the above-mentioned problems, Japanese Patent No. 3,359,114 discloses a use of a thin ND filter obtained by vacuum film formation. The thin ND filter is formed of a multi-layered film in which a metal oxide layer and a dielectric layer are stacked. In this case, the metal oxide layer is used as an absorbing layer with respect to incident light. Further, the thin film ND filter exhibits a function as a reflection preventing film by adjusting thicknesses of the metal oxide layer and the dielectric layer.
In recent years, with further miniaturization of a camera and configuration of a sensor as a charge-coupled device (CCD), a positional relationship between a lens system and a CCD sensor tends to approach each other. Therefore, there arises a problem in that return light reflected when a part of incident light is reflected by a CCD plane becomes ghost or flare to influence image quality. Specifically, the ND filter is required to be excellent in rear surface reflection preventing characteristics, as well as flat transmittance characteristics and front surface reflection preventing characteristics. However, among the thin film ND filters described in Japanese Patent No. 3,359,114, those of a type having a multi-layered film on one surface are insufficient in terms of the rear surface reflection preventing characteristics, although they are excellent in front surface reflection preventing characteristics. Further, in the ND filters of a type having a multi-layered film on both surfaces, the rear surface reflection preventing characteristics can be obtained, but the number of film formation and costs are doubled. As a result, those cannot be realized in terms of production.
Japanese Patent Application Laid-Open No. 2003-344612 discloses an example of an ND filter that realizes satisfactory front surface reflection preventing characteristics and rear surface reflection preventing characteristics, and a transmittance of 10% or less. However, in Japanese Patent Application Laid-Open No. 2003-344612, TiO2, Nb2O5, ZrO2, and the like used as examples of materials having a refractive index of 2.0 or more are generally used for film formation in an oxygen atmosphere so as to secure a stability of a refractive index and transparency. In this case, an optical constant of the light absorbing film changes due to a change in an oxygen partial pressure after film formation and the like, and in particular, a large variation occurs in the rear surface reflection.
In order to address a degradation or the like in image quality caused by a diffraction due to a small diaphragm accompanied with high sensitivity and high integration of a recent imaging element, there is a demand for an ND filter of a high concentration with an optical concentration of about 1 or more, i.e., a transmittance of about 10% or less. Therefore, it is necessary to increase a total thickness of an absorbing film for absorbing incident light.
In general, a thickness of each layer of the absorbing film may be increased so as to address the ND filter of a high concentration. However, if the absorbing film is increased in thickness, it is difficult to obtain a reflection preventing effect using light interference as in a multi-layered transparent optical thin film because incident light is attenuated in each layer. Further, the ND filter is likely to be influenced by a change in optical constants (refractive index and absorption coefficient) of the light absorbing film itself, whereby optical performance is not stable. Thus, there is also a problem in that flatness of transmitted light is difficult to be maintained due to wavelength dependency of the absorption coefficient of the absorbing film.
In order to solve the above-mentioned problems, it is possible to divide the absorbing film into a plurality of layers, thereby ensuring the reflection preventing effect and the transmittance flatness. However, in this case, the entire number of layers increases to a large extent, whereby cost increases due to increase in a film-formation time. Further, the layer configuration designed considering the front surface reflection preventing characteristics does not satisfy the rear surface reflection preventing characteristics, so that the rear surface reflection preventing characteristics become insufficient.